Voice operated switches for activating the transmission or record mode of operation on headsets, walkie talkies, two way radios, tape recorders, and the like are well known.
Such voice operated switches function by measuring the output of a microphone and only providing that output to amplification circuitry when the output exceeds a predetermined threshold level. Thus, ideally, transmission only takes place when the speaker is talking. Such operation is particularly useful in half-duplex systems, wherein a transmission can only be received when the listener is not talking.
The use of a voice operated switch (VOX) also mitigates power consumption, since the transmitter is not turned on all of the time. It may also reduce stress upon anyone listening to the transmission, since ambient noise is not transmitted when the person operating the transmitter is not speaking (at least in ideal situations). Voice operated switches are also useful when it is difficult or undesirable to manually actuate a transmitter, such as by pushing a microphone button. This may be the case when the user's hands are otherwise occupied. A tank driver, for example, must frequently use both hands to control the tank.
However, one problem commonly associated with voice operated switches is that they are susceptible to being inadvertently actuated by high ambient noise levels. As those skilled in the art will appreciate, high ambient noise levels frequently result in undesirable triggering of the voice operated switch, thereby causing the transmit channel to remain open when the speaker is not actually talking. Of course, holding the transmit channel open on a high noise environment results in the transmission of the noise to the receiver. It can be very annoying for the a listener at the receiver to listen to such noise and may, in fact be dangerous if it distracts the listener, in some instances.
Although the sensitivity of such voice operated switches may be varied, so as to mitigate undesirable triggering by high ambient noise, such changing of the threshold inherently makes it more difficult for the user to trigger the voice operated switch by talking. That is, the user must talk at a higher level in order to trigger a less sensitive voice operated switch, particularly for switches such as this one, which relies primarily on energy level as opposed to spectral components. This may be undesirable since it may not be natural, and is easy to forget, thereby resulting in non-transmitted communications. It also may be uncomfortable for the user to talk at such a higher level. Talking at a higher level for an extended length of time may result in hoarseness or fatigue, or even a temporary loss of voice.
Most contemporary voice operated switch algorithms are designed specifically for low noise environments, where a large increase in overall signal power is actually a very reliable indicator of the presence of the speakers voice. Although algorithms which are specifically designed for high noise environments are known, such high noise voice operated switch algorithms typically employ Fast Fourier Transforms (FFT) or floating-point digital filters, both of which require a significant amount of processing power.
A particular problem with voice operated switches occurs when the ambient noise level is frequently changing and is sometimes very high. In these instances, it is not practical to continually readjust the sensitivity of the microphone trigger circuit. Thus, it is desirable to provide an adaptive voice operated switch algorithm which works effectively in changing high noise environments, particularly wherein the algorithm does not require excessive processing power.